Synthesis of (+)-conagenin

Article abstract of DOI:10.1021/jo050407s

An efficient total synthesis of (+)-conagenin was achieved. The right fragment of conagenin, α-methylserine containing a quaternary stereocenter attached to nitrogen, was synthesized using allyl cyanate-to-isocyanate rearrangement. The left fragment, 2,4-

Full text of DOI:10.1021/jo050407s

SCHEME 1
Synthesis of (+)-Conagenin
Yohei Matsukawa,† Minoru Isobe,†
Hiyoshizo Kotsuki,‡ and Yoshiyasu Ichikawa*,‡
Laboratory of Natural Products, Faculty of Science, Kochi
University, Akebono-cho, Kochi 780-8520, Japan, and
Laboratory of Organic Chemistry, School of Bioagricultural
Sciences, Nagoya University, Nagoya 464-8601, Japan
SCHEME 2a
ichikawa@cc.kochi-u.ac.jp
Received March 3, 2005
An efficient total synthesis of (+)-conagenin was achieved.
The right fragment of conagenin, R-methylserine containing
a quaternary stereocenter attached to nitrogen, was syn-
thesized using allyl cyanate-to-isocyanate rearrangement.
The left fragment, 2,4-dihydroxy-3-methylpentanoic acid,
has three contiguous stereogenic centers, which was ef-
ficiently constructed by enantioselective monoreduction of
2-alkyl-1,3-diketones reported by Cossy, and chelation-
controlled stereoselective reduction of â-hydroxy ketone.
These two fragments were coupled through intramolecular
amide bond formation with high efficiency.
a Reagents and conditions: (a) TBDPSCl, imidazole, DMF, 95%;
(b) DIBAL, CH2Cl2, -78 °C; (c) Ph3PC(CH3)COOEt, CH2Cl2, 85%,
two steps; (d) DIBAL, CH2Cl2, -78 °C, 85%; (e) TrCl, pyridine, 70
°C, 98%; (f) Bu4NF, CH3CN, 60 °C, 95%; (g) CCl3CONCO, CH2Cl2
then aq K2CO3/MeOH, quant; (h) PPh3, CBr4, Et3N, CH2Cl2, -10
°C; (i) PhCH2ONa, MS 4A, THF, 90%; two steps; (j) OsO4, NMO,
acetone/H2O; (k) NaIO4, THF/H2O, 97%, two steps; (l) NaClO2,
2-methyl-2-butene, KH2PO4, t-BuOH/H2O; (m) MeI, K2CO3, DMF,
87%, two steps; (n) TFA, CH2Cl2, 92%.
Conagenin (1) is a unique biologically important sec-
ondary metabolite isolated from the culture broths of
Streptomyces roseosporus by Ishizuka and co-workers.1
This compound stimulates activated T cells as a low
molecular weight immunomodulator and was found to
improve the antitumor efficacy of adriamycin and mito-
mycin C against murine leukemias, which suggest its
potential utility for cancer chemotherapy.2,3 Not surpris-
ingly, therefore, conagenin has been an attractive target
for the synthetic chemist, and two total syntheses of
1 have now been reported,4a,b in addition to formal
synthesis4c and synthesis of its analogues.5
with [3.3] sigmatropic rearrangement of allyl cyanate as
represented in Scheme 2.6
The hydroxyl group of D-lactic acid methyl ester (4) was
protected as t-butyldiphenylsilyl (TBDPS) ether (95%
yield). Reduction of the ester 5 with diisobutylaluminum
hydride (DIBAL) followed by chain extension using ethyl
2-(triphenylphosphoranylidene)propionate furnished un-
saturated ester 6 exclusively in 85% yield over two steps.7
Treatment of the ester 6 with DIBAL in dichloromethane
at -78 °C afforded allyl alcohol 7 (85% yield), and
protection of the hydroxyl group in 7 as a triphenylmethyl
ether and removal of the TBDPS group with tetrabutyl-
ammonium fluoride gave allyl alcohol 8 (93% yield, over
two steps). This allyl alcohol 8 was then transformed into
The synthesis of conagenin was to be assembled from
two fragments 2 and 3 (Scheme 1), and a nitrogen-
substituted quaternary stereocenter in the right R-me-
thylserine moiety 3 was stereoselectively constructed
(4) (a) Hatakeyama, S.; Fukuyama, H.; Mukugi, Y.; Irie, H. Tetra-
hedron Lett. 1996, 37, 4047. (b) Sano, S. Miwa, T.; Hayashi, K.; Nozaki,
K.; Ozaki, Y.; Nagao, Y. Tetrahedron Lett. 2001, 42, 4029. Formal
asymmetric synthesis: (c) Enders, D.; Bartsch, M.; Runsink, J.
Synthesis 1999, 243.
† Nagoya University.
‡ Kochi University.
(1) Yamashita, T.; Iijima, M.; Nakamura, H.; Isshiki, K.; Naganawa,
H.; Hattori, S.; Hamada, M.; Ishizuka, M.; Takeuchi, T.; Iitaka, Y. J.
Antibiot. 1991, 44, 557.
(2) (a) Kawatsu, M.; Yamashita, T.; Osono, M.; Ishizuka, M.;
Takeuchi, T. J. Antibiot. 1993, 46, 1687. (b) Kawatsu, M.; Yamashita,
T.; Ishizuka, M.; Takeuchi, T. J. Antibiot. 1994, 47, 1123. (c) Ishizuka,
M.; Kawatsu, M.; Yamashita, T.; Ueno, M.; Takeuchi, T. Int. J.
Immunopharmacol. 1995, 17, 133.
(3) (a) Kawatsu, M.; Yamashita, T.; Ishizuka, M.; Takeuchi, T. J.
Antibiot. 1995, 48, 222. (b) Hamada, M.; Yamamoto, S.; Moriguchi,
S.; Kishino, Y. J. Antibiot. 2001, 54, 349. (c) Hamada, M.; Sonotake,
E.; Yamamoto, S. Moriguchi, S. J. Antibiot. 1999, 53, 548.
(5) Diastereomers synthesis: (a) Kova´cs-Kulya´ssa, AÄ .; Herczegh, P.;
Sztaricskai, F. J. Tetrahedron Lett. 1996, 37, 4047. (b) Kova´cs-
Kulya´ssa, AÄ .; Herczegh, P.; Sztaricskai, F. J. Tetrahedron 1997, 53,
13883. Synthetic study: (c) Rodrigues, J. A. R.; Moran, P. J. S.; Milagre,
C. D. F.; Ursini, C. V. Tetrahedron Lett. 2004, 45, 3579.
(6) (a) Ichikawa, Y. Synlett 1991, 238. (b) Ichikawa, Y.; Tsuboi, K.;
Isobe, M. J. Chem. Soc., Perkin Trans. 1 1994, 2791. (c) Ichikawa, Y.;
Osada, M.; Ohtani, I.; Isobe, M. J. Chem. Soc., Perkin Trans. 1 1997,
1449.
(7) (Z)-Isomer was not detectable by 1H NMR analysis.
10.1021/jo050407s CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/25/2005
J. Org. Chem. 2005, 70, 5339-5341
5339

SCHEME 3a
SCHEME 4a
a Reagents and conditions: (a) DCC, DMAP, HOBt, CH2Cl2; (b)
H2, 10% Pd-C, THF; (c) aq NaHCO3, 90%, three steps; (d) 1 M
K2CO3, MeOH, 87%.
a Reagents and conditions: (a) Ac2O, TsOH, 2BF3‚3AcOH, 89%;
(b) 15, HCOOH, Et3N, CH2Cl2, 40 °C, 89%; (c) Zn(BH4)2, Et2O, 0
°C, 70%; (d) Ac2O, DMAP, pyridine, 98%; (e) RuCl3‚nH2O, H5IO6,
CCl4/CH3CN/H2O, 78%.
from hexane (70% yield). After protection of the two
hydroxyl groups in 17 as acetates (Ac2O, DMAP, pyri-
dine), the phenyl group was oxidatively cleaved with
ruthenium tetraoxide using periodic acid as the stoicheo-
metric oxidant in a biphasic system (CCl4/CH3CN/H2O)
to afford the left fragment of conagenin 2 (76% yield in
two steps).14
Having obtained two fragments, pentanoic acid 2 and
R-methylserine 3, we next turned our attention to the
amide bond construction by an intramolecular ester-to-
amide exchange reaction. Condensation of 2 with 3 was
carried out cleanly using dicyclohexylcarbodiimide (DCC)
in the presence of DMAP and 1-hydroxybenzotriazole
(HOBt) in dichloromethane to afford ester 18 (Scheme
4). Removal of the Cbz group by hydrogenolysis (H2, 10%
Pd-C, THF) and subsequent treatment of the resultant
ester 19 with aqueous sodium bicarbonate gave rise to
the amide 20 in 90% yield over three steps.15 Finally,
hydrolytic removal of the two acetyl and methyl ester
protecting groups (1M K2CO3, MeOH) furnished (+)-
conagenin (1), which exhibited spectroscopic and physical
properties identical to those of the authentic samples.16
allyl carbamate 9 by treatment with trichloroacetyl
isocyanate followed by hydrolysis of the resultant N-
trichloroacetyl carbamate with potassium carbonate in
aqueous methanol. Dehydration of allyl carbamate 9 was
carried out by using the modified Appel conditions (PPh3,
CBr4, Et3N, CH2Cl2, -10 °C)8 to generate allyl cyanate
10, which immediately underwent [3.3] sigmatropic rear-
rangement at -10 °C to afford allyl isocyanate 11. After
careful workup, the resultant isocyanate 11 was im-
mediately treated with sodium benzyl alkoxide in THF.
Benzyl carbamate 129 was obtained in 90% overall yield
from 8. Transformation of the double bond in 12 into the
methoxycarbonyl group was accomplished in 84% overall
yield by a four-step sequence: (i) catalytic osmylation
with N-methylmorpholine N-oxide, (ii) diol cleavage using
sodium periodate, (iii) sodium chlorite oxidation of the
resultant aldehyde, and (iv) esterification with methyl
iodide and potassium carbonate in DMF. Finally, removal
of triphenylmethyl group with trifluoroacetic acid in
dichloromethane gave the requisite R-methylserine 3).
For the synthesis of left fragment of conagenin (Scheme
3), it was envisioned that Hatakeyama’s intermediate
174a could be expeditiously accessed from 2-methyl-1,3-
diketone 14, which was prepared by Lewis acid catalyzed
acylation of propiophenone (13) in 89% yield. Following
the protocol reported by Cossy, enantioselective monore-
duction of 14 using chiral ruthenium catalyst, (S,S)-Ru-
[N-(tosyl)-1,2-diphenylethylenediamine] (p-cymene) (15),10
proceeded smoothly to afford syn-2-methyl-3-hydroxyke-
tone 16 in good 89% yield.11 The chelation-controlled
reduction of 16 using zinc borohydride in ether12 gave
the desired syn-1,3-diol 17 (syn/anti f 30:1),13 which was
purified by column chromatography and recrystallization
Experimental Section
(1R,2E)-1,3-Dimethyl-4-(trityloxy)but-2-en-1-yl carbam-
ate (9). To a solution of allyl alcohol 8 (270 mg, 0.75 mmol) in
CH2Cl2 (5.0 mL) at 0 °C was added trichloroacetyl isocyanate
(13) The stereochemistry of this reduction, initially assigned on the
basis of a chelation-control model, was confirmed by 13C NMR analysis
of the acetonide i derived from 1,3-diol 17. In general, the syn-1,3-diol
acetonides have carbon resonances for the acetonide methyl groups at
30 and 19 ppm, while the anti-isomers show methyl resonances around
25 ppm. The 13C NMR spectra of i showed two methyl signals at 19.6
and 30.1 ppm, which unambiguously determined the configuration of
17. See: (a) Rychnovsky, S. D.; Skalitzky, D. J. Tetrahedron Lett. 1990,
31, 945. (b) Evans, D. A.; Rieger, D. L.; Gage, J. R. Tetrahedron Lett.
1990, 31, 7099..
(8) (a) Ichikawa, Y. J. Chem. Soc., Perkin Trans. 1 1992, 2135. (b)
O’Neil, I. A. In Comprehensive Organic Functional Group Transforma-
tions; Katritzky, A., Meth-Cohn, O., Ress, C. W., Eds.; Pergamon:
Oxford, 1995; Vol. 3, p 696.
(9) Enantiomeric purity of 12 was determined to be 97% ee by 1H
NMR analysis of the MTPA esters, which were prepared by removal
of triphenylmethyl group in 12 followed by esterification with (+)- and
(-)-MTPA chlorides. This result indicates that [1.3]-chirality transfer
of allyl cyanate 10 was achieved with excellent selectivity.
(10) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R.
J. Am. Chem. Soc. 1995, 117, 7562.
(14) (a) Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K.
B. J. Org. Chem. 1981, 46, 3936. (b) Nu-ez, M. T.; Martin, V. S.; J.
Org. Chem. 1990, 55, 1928.
(15) Similar intramolecular acyl migration using byproduct was
noted in the previous synthesis of conagenin, and our experiments
followed its procedure. See ref 4a.
(11) This highly enantio- and diastereoselective reduction of 14 has
been reported by Cossy. See: Eustache, F.; Dalko, P. I.; Cossy, J. Org.
Lett. 2002, 4, 1263.
(16) The NMR analysis of synthetic conagenin (1) in D2O was
complicated by sample dependent variability in the chemical shifts of
the methylene protons in the R-methylserine unit. This problem was
avoided by measurement of the spectrum in CD3OD.
(12) Nakata, T.; Tani, Y.; Hatozaki, M.; Oishi, T. Chem Pharm. Bull.
1984, 32, 1411.
5340 J. Org. Chem., Vol. 70, No. 13, 2005

(180 µL, 1.51 mmol), and the reaction mixture was stirred at 0
°C for 30 min. To this solution was added an aqueous potassium
carbonate (410 mg in 1.0 mL of water) and MeOH (4.0 mL), and
CH2Cl2 was removed by evaporation. The reaction mixture was
stirred at room temperature for 2 h and then diluted with AcOEt.
The mixture was washed with H2O, and the combined aqueous
layer was extracted with AcOEt. The combined organic layers
were washed with brine, dried over Na2SO4, and concentrated
under reduced pressure. The crude product was purified by
column chromatography on silica gel eluting with AcOEt/hexane
(1:4 and 1:3) to afford allyl carbamate 9 (300 mg, quant) as a
After being stirred at room temperature for 1 h, the reaction
mixture was filtered and concentrated under reduced pressure.
Purification by column chromatography on silica gel eluting with
AcOEt/hexane (1:3) gave crude ester 18, which was dissolved in
THF (2.0 mL). Palladium on carbon (10%, 10 mg) was added,
and the solution was stirred vigorously under hydrogen atmo-
sphere for 30 min. After the deprotection of the Cbz-group in
18 was checked by TLC analysis, aqueous NaHCO3 (0.20 mL)
was added. After being stirred at room temperature for 10 h,
the reaction mixture was filtered and concentrated under
reduced pressure. Purification by column chromatography on
silica gel eluting with AcOEt/hexane (1:1 and 2:1) gave the
protected conagenin (20) (35 mg, 90%, three steps) as a clear
gum: [R]21D ) +32.9 (c 0.35, CHCl3); IR (KBr) νmax ) 3404, 1739,
colorless gum: [R]17 ) +12.1 (c 1.92, CHCl3); IR (KBr) νmax
)
D
3486, 3347, 1718 cm-1; 1H NMR (CDCl3, 400 MHz): δ 1.32 (3H,
d, J ) 6.0), 1.67 (3H, s), 3.50 (2H), 4.62 (2H), 5.56 (1H, dq, J )
9.0, 6.5), 5.62 (1H, d, J ) 9.0), 7.20-7.47 (15H); 13C NMR (CDCl3,
100 MHz): δ 14.5, 21.1, 68.1, 68.5, 86.8, 124.7, 127.0, 127.8,
128.6, 136.2, 144.1, 156.5; HRMS (FAB) calcd for C26H28NO3 [M
+ H]+ 402.2069, found 402.2057.
1
1683 cm-1; H NMR (CDCl3, 400 MHz) δ 1.02 (3H, d, J ) 7.1),
1.26 (3H, d, J ) 6.3), 1.55 (3H, s), 2.06 (3H, s), 2.18 (3H, s), 2.28
(1H, qt, J ) 7.1, 5.4), 3.27 (1H), 3.80 (3H, s), 3.82 (1H, dd, J )
11.0, 6.5), 4.13 (1H, dd, J ) 11.0, 4.5), 4.99 (1H, d, J ) 11.5),
5.02 (1H, qd, J ) 6.3, 5.4), 7.14 (1H, s); 13C NMR (CDCl3, 100
MHz) δ 9.6, 18.0, 19.6, 20.8, 21.2, 39.7, 53.0, 62.4, 65.4, 71.0,
Benzyl {(1R,2E)-1-Methyl-1-[(trityloxy)methyl]but-2-en-
1-yl}carbamate (12). To a solution of allyl carbamate 9 (96 mg,
0.24 mmol), triphenylphosphine (157 mg, 0.60 mmol), and
triethylamine (120 µL, 0.87 mmol) in CH2Cl2 (3.0 mL) cooled to
-10 °C was added a solution of carbon tetrabromide (222 mg,
0.67 mmol) in CH2Cl2 (0.50 mL). After being stirred at -10 °C
for 30 min, the reaction mixture was diluted with hexane (20
mL). The resulting reaction mixture was washed with H2O, 1
M KHSO4, aqueous saturated NaHCO3, and brine and dried over
Na2SO4. Concentration under reduced pressure gave crude allyl
isocyanate 11, which was immediately dissolved in THF (2.0
mL). Powdered molecular sieves 4A were added to the solution,
and the resulting suspension was treated at 0 °C with a solution
of sodium benzyl alkoxide [prepared from benzyl alcohol (52 mg,
0.48 mmol) and sodium hydride (20 mg, 60% dispersion in
mineral oil, 0.50 mmol) in 1.0 mL of THF]. After being stirred
at room temperature for 1 h, the reaction mixture was diluted
with AcOEt, washed with saturated NH4Cl and brine, and dried
over Na2SO4. Concentration under reduced pressure and puri-
fication by column chromatography on silica gel eluting with
AcOEt/hexane (0:1 and 1:20) furnished benzyl carbamate 12 (100
mg, 85%) as a colorless syrup: [R]20D ) +5.7 (c 1.00, CHCl3); IR
75.1, 168.9, 170.4, 170.9, 173.5; HRMS (FAB) calcd for C15H26
-
NO8 [M + H]+ 348.1658, found 348.1667.
(+)-Conagenin (1). To a solution of protected conagenin (20)
(35 mg, 0.10 mmol) in MeOH (1.20 mL) cooled to 0 °C was added
aqueous K2CO3 (1.0 M, 0.40 mL), and the cooling bath was
removed. After being stirred at room temperature for 2 h, the
reaction mixture was neutralized with aqueous KHSO4 (1.0 M,
1.0 mL). The reaction mixture was concentrated under reduced
pressure to afford the residue, which was purified by ODS
column chromatography (Cosmosil 75 C18-OPN, H2O followed
by 19:1 H2O/MeCN as eluent) to furnish (+)-conagenin (22 mg,
87%) as a colorless crystal: mp 153-155 °C; [R]20 ) +56.8 (c
D
0.44, MeOH) (lit.1 [R]27 ) +55.4); IR (KBr) νmax ) 3488, 3369,
D
1
3325, 3059, 1703, 1635, 1530, 1457, 1250 cm-1; H NMR (CD3-
OD, 400 MHz) δ 0.94 (3H, d, J ) 7.0), 1.22 (3H, d, J ) 6.0), 1.51
(3H, s), 1.89 (1H, qdd, J ) 7.0, 6.0, 2.5), 3.82 (1H, d, J ) 11.0),
3.85 (1H, quint, J ) 6.0), 4.02 (1H, d, J ) 11.0), 4.16 (1H, d, J
) 2.5), 8.10 (1H, s, exchanged with CD3OD); 13C NMR (CD3OD,
100 MHz) δ 8.2, 19.9, 21.2, 43.7, 62.5, 66.0, 71.2, 75.2, 175.8,
176.4; HRMS (FAB) calcd for C10H20NO6 [M + H]+ 250.1291,
found 250.1286.
(KBr) νmax ) 3428, 3354, 1731 cm-1 1H NMR (CDCl3, 400
;
MHz): δ 1.44 (3H, s), 1.68 (3H, d, J ) 4.4), 3.05 (1H, d, J ) 9.0),
3.13 (1H, d, J ) 9.0), 5.04 (2H, s), 5.14 (1H, s), 5.46-5.60 (2H),
7.20-7.44 (20H); 13C NMR (CDCl3, 100 MHz): δ 17.8, 22.3, 56.4,
66.1, 69.0, 86.5, 124.6, 127.0, 127.8, 127.9, 128.0, 128.4, 128.7,
133.8, 136.7, 143.7, 154.7; HRMS (FAB) calcd for C33H34NO3 [M
+ H]+ 492.2534, found 492.2527.
Acknowledgment. We thank Professor S. Hatakeya-
ma (Nagasaki University) for providing a sample of (+)-
conagenin and its related spectra. This work was
supported by a grant from Fujisawa Foundation.
Methyl N-[(2R,3S,4R)-2,4-Diacetyloxy-3-methylvaleryl]-
2-methyl-L-serinate (20). Dicyclohexylcarbodiimide (35 mg,
0.17 mmol) was added to a solution of R-methylserine (3) (30
mg, 0.11 mmol), pentanoic acid (2) (30 mg, 0.13 mmol), and
HOBt (23 mg, 0.17 mmol) in CH2Cl2 (1.0 mL) at room temper-
ature. After the mixture was stirred for 30 min, 4-(dimethyl-
amino)pyridine (16 mg, 0.13 mmol) was added in one portion.
Supporting Information Available: Experimental pro-
cedures and spectral data for all relevant compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO050407S
J. Org. Chem, Vol. 70, No. 13, 2005 5341